Multiple Realizability

In the philosophy of mind, the multiple realizability thesis
contends that a single mental kind (property, state, event) can be
realized by many distinct physical kinds. A common example is pain.
Many philosophers have asserted that a wide variety of physical
properties, states, or events, sharing no features in common at that
level of description, can all realize the same pain. This thesis served
as a premise in the most influential argument against early theories
that identified mental states with brain states (psychoneural identity
theories). It also served in early arguments for functionalism.
Nonreductive physicalists later adopted it (usually without alteration)
to challenge all varieties of psychophysical reductionism. The argument
has even been employed to challenge the functionalism it initially
motivated.

Reductionists have offered numerous responses. Initial responses
either attacked the argument from the multiple realizability premise to
the anti-reduction/identity theory conclusion or proposed revisions to
classical reductionism that accommodated the premise. More recently,
some reductionists have questioned the truth of the multiple
realizability premise itself.

The multiple realizability thesis about the mental is that a given
psychological kind (like pain) can be realized by many distinct
physical kinds: brain states in the case of earthly mammals, electronic
states in the case of properly programmed digital computers, green
slime states in the case of extraterrestrials, and so on. Correctly
characterizing the realization relation remains a contentious matter in
analytic metaphysics (Gillett 2003, Polger 2004). But whatever the
correct account turns out to be, the multiple realizability thesis
about the mental is that a given psychological kind (like pain) can
stand in that relationship to many distinct physical kinds.

In a pair of examples illustrating multiple realizability in special
sciences (economics and psychology), Jerry Fodor (1974) implicitly
distinguished between two types of the relation. Call the first type,
illustrated in the examples provided at the end of the previous
paragraph, multiple realizability “over physical structure-types”:
creatures with distinct physical structures realizing their
psychological states can nevertheless entertain the same psychological
states. A more radical type of multiple realizability would obtain if a
token physical (e.g., nervous) system can realize a single mental kind
via distinct physical states of that same system at different times.
Call this second sense multiple realizability “in a token system over
times.” (These terms are from John Bickle 1998, Chapter 4.) This second
sense is more radical because there could be a disjunction of physical
states realizing each mental kind for every existing cognizer. The
importance of the more radical type is discussed further (section 1.5
below).

In a series of papers published throughout the 1960s, Hilary Putnam
introduced multiple realizability into the philosophy of mind. Against
the “brain state theorists,” who held that every mental kind is
identical to some as-yet-undiscovered neural kind, Putnam (1967) notes
the wide variety of terrestrial creatures seemingly capable of
experiencing pain. Humans, other primates, other mammals, birds,
reptiles, amphibians, and even mollusks (e.g., octopi) seem reasonable
candidates. But then for the “brain state theory” to be
true, there must be some physical-chemical kind common to this wide
variety of pain-bearing species, and correlated exactly with each
occurrence of the mental kind. (This is a necessary condition of the
hypothesized type-identity.) But comparative neuroanatomy and
physiology, facts about convergent evolution, and the corticalization
of function (especially sensory function) as cortical mass increases
across species all speak against this requirement.

In addition, early mind-brain identity theorists insisted that these
identities, while contingent, hold by virtue of natural (scientific)
law. But then any physically possible cognizer (e.g., pain-bearer) must
also be capable of possessing that physical-chemical kind. Here the
well-known philosophers' fantasies enter the discussion. Silicon-based
androids, artificially intelligent electronic robots, and Martians with
green slime pulsating within their skulls all seem to be possible pain
realizers. But they lack “brain states” comparable to ours
at any level of physical description. Further still, these mind-brain
identity theories were supposed to be completely general.
Every mental kind was held to be identical to some neural
kind. So the critic needs to find only one mental kind, shared across
species yet realized differently at the physical-chemical level. Putnam
acknowledges that the early identity theories were empirical
hypotheses. But one of their consequences was “certainly ambitious” and
very probably false.

If a given mental kind is multiply realizable by distinct physical
kinds, then it cannot be identical to any specific physical kind.

(the anti-identity thesis conclusion) No mental kind is identical
to any specific physical kind.

In this simple form, this is a deductively valid argument.

Fodor (1974) extended Putnam's initial argument by arguing
that reductionism imposes too strong a constraint on acceptable
theories in special sciences like psychology. According to Fodor,
reductionism is the conjunction of “token physicalism” with the claim
that there are natural kind predicates in an ideally completed physics
corresponding to each natural kind term in any ideally completed
special science. He characterized “token physicalism” in turn as the
claim that all events that science talks about are physical
events—a weaker thesis than reductionism or type-type
physicalism. Consider the following string of numerals:

1 1 2.

This string contains two types of numerals (1 and 2), but three
tokens of the two types (two tokens of the numeral type 1 and one token
of the numeral type 2). Mental states admit of a similar ambiguity.
When you and I both entertain the belief that Fodor advocates a
Language of Thought, one type of mental state is entertained, but two
tokens of that type (your belief state and my belief state). Type-type
physicalism insists that types of mental states are identical to types
of physical states; this view runs afoul of multiple realizability. But
token physicalism only insists that each token occurrence of each type
of mental state is identical to some token occurrence of a physical
state type—not necessarily an occurrence of a token of the same
physical state type on each occasion.

Fodor gave reductionists the best-developed theory of reduction at
the time: Ernest Nagel's (1961) “derivability” account of
intertheoretic reduction. Nagel's account “connects” disparate
elements of the reduced and reducing theories' vocabularies via
“bridge laws” (not Nagel's term!) and claims a reduction when the
laws of the reduced theory are derived from the laws of the reducing
and the bridge laws. According to Fodor (1974), if reductionism is to
establish physicalism, these cross-theoretic bridge laws must assert
(contingent) identities of reduced and reducing kinds. But given
multiple realizability, the only way this can obtain is if the physical
science constituent of a psychophysical bridge law is a disjunction of
all the terms denoting possible physical realizations of the mental
kind. Given the extent and variety of actual (not to say possible)
physical realizations, it is overwhelmingly likely that the disjunctive
component will not be a kind-predicate of any specific physical
science. It is also overwhelmingly likely that the disjunctive
component will not appear in any genuine law of a specific physical
science. Multiple realizability thus demonstrates that the additional
requirement of reductionism (beyond token physicalism) is empirically
untenable.

The multiple realizability premise has also been used, albeit more
indirectly, in early arguments for functionalism. Functionalism in the
philosophy of mind individuates mental states in terms of their causes
and effects. Pain, for example, is caused by tissue damage or trauma to
bodily regions, and in turn causes beliefs (e.g., that one is in pain),
desires (e.g., that one relieves the pain), and behaviors like crying
out, nursing the damaged area, and seeking out pain relieving drugs.
Any internal state that mediates a similar pattern of causes and
effects is pain—regardless of the specific physical mechanisms
that mediate the pattern in any given case. Ned Block and Jerry Fodor
(1972) note that the multiple realizability of mental on physical types
shows that any physicalist type-identity hypothesis will fail to be
sufficiently abstract. Functionalism, on the other hand, seems to be at
the next level of abstraction up from explanation of behavior based on
physical mechanisms. In addition, it seems sufficiently abstract to
handle multiple realizability. Block and Fodor also note that multiple
realizability at the level of physical description is a common
characteristic of ordinary functional kinds, like mousetraps and valve
lifters. Characterizing mental kinds as functional kinds thus appears
to be at exactly the right level of abstraction to handle multiple
realizability. It is a reasonable empirical hypothesis in light of this
feature of mental states.

Notice that this argument for functionalism is explicitly
non-deductive, in contrast to the deductive (and valid) nature of
Putnam's original argument against identity theories. It is
important to keep the anti-identity theory argument separate from the
pro-functionalism argument, as some criticisms of multiple
realizability may be telling against one but irrelevant against the
other.

Many contemporary nonreductive materialists deny that mental kinds
can be identified with functional kinds. Some of their criticisms of
functionalism hinge on issues about individualism in psychology. But
Putnam has used multiple realizability to argue against functionalism
itself. In specifying the nature of mental kinds, many functionalists
followed Putnam (and Fodor) by adopting “Turing machine functionalism”:
mental kinds are identical to the computational kinds of a suitably
programmed universal Turing machine. Putnam (1988), however, has argued
that mental kinds are both “compositionally” and “computationally”
plastic. The first point is his familiar multiple realizability
contention of the mental on the physical. The second contends that the
same mental kind can be a property of systems that are not in the same
(Turing) computational state. In this work, multiple realizability
strikes back at the very theory of mind it initially motivated.

Psychologist Zenon Pylyshyn (1984) appeals to multiple realizability
to ground a methodological criticism of reductionism. He described a
pedestrian, having just witnessed an automobile accident, rushing into
a nearby phone booth and dialing a 9 and a 1. What will this person do
next? Dial another 1, with overwhelming likelihood. Why? Because of a
systematic generalization holding between what he recognized, his
background knowledge, his resulting intentions, and that action
(intentionally described).

We won't discover that generalization, however, if we focus on the
person's neurophysiology and resulting muscular contractions.
That level of explanation is too weak, for it cannot tell us that this
sequence of neural events and muscular contractions corresponds to the
action of dialing a 1. A given physiological explanation only links one
way of learning the emergency phone number to one way of coming to know
that an emergency occurred to one sequence of neural events and
resulting muscular contractions producing the behavior
(nonintentionally described). However, the number of physical events
constituting each of these cognitive classes—the learning, the
coming to know, and the action of dialing—is potentially
unlimited, with the constituents of each class often unrelated to each
other at the physiological level of description. (This is
Pylyshyn's appeal to multiple realizability.) So if there is a
generalization at the higher level of description available for
capturing (and in the pedestrian example there surely is), an
exclusively reductionist approach to psychological explanation will
miss it. Thus because of multiple realizability, reductionism violates
a tenet of scientific methodology: seek to capture all capturable
generalizations. (Fodor 1975, Chapter 5, and Terence Horgan 1993 raise
related methodological caveats about reductionism resting ultimately on
multiple realizability. Bickle 1998, Chapter 4, responds to these.)

Recent anti-reductionists have stressed the more radical type of
multiple realizability, in a token system over times. As far back as
the late 1970s, Block (1978) insisted that the required narrowing of
psychological kinds due to the more radical type of multiple
realizability renders psychology incapable of capturing whatever
generalizations hold across species. Ronald Endicott (1993) gives
Block's reply an empirical twist by noting detailed facts about
plasticity in individual human brains. The capacity for distinct neural
structures and processes to subserve a given psychological function
owing to trauma, damage, changing task demands, development, and other
factors is extensive. These facts count further against any reduction
of or identities between psychological and physical kinds. Horgan
(1993) clearly appeals to this radical sense of multiple realizability
when he writes:

Multiple realizability might well begin at home. For all we now know
(and I emphasize that we really do not now know), the
intentional mental states we attribute to one another might turn out to
be radically multiply realizable at the neurobiological level of
description, even in humans; indeed, even in
individual humans; indeed, even in an individual human
given the structure of his central nervous system at a single
moment of his life. (p. 308; author's emphases)

This radical sense has become to default position for nonreductive
physicalists, whose solution to the mind-body problem still dominates
Anglo-American philosophy of mind. Putnam's original multiple
realizability remains central to this solution, with the second premise
now replaced with:

(2′) If mental kinds are multiply realizable (in the radical
sense), then psychology cannot be reduced to a physical science;

Robert Richardson (1979) suggests that the Putnam-Fodor challenge to
reductionism results from a misunderstanding of Ernest Nagel's actual
account of intertheoretic reduction. Although Nagel's detailed examples
of historical cases all involve biconditional cross-(reduced and
reducing) theory “conditions of connectivity,” one-way
conditional connections expressing sufficient conditions at the
reducing level are all that his “principle of derivability” requires.
Richardson even cites passages from Nagel (1961) indicating that Nagel
himself saw this point. Multiple realizability only challenges
necessity (and nondisjunctive) reducing conditions, and so is not a
challenge to even a projected Nagelian reduction of psychology to the
physical sciences.

David Lewis (1969) argues that the inconsistency between the
reductionist's thesis and multiple realizability evaporates when we
notice a tacit relativity of the former to contexts. A common sense
example illustrates his point. The following three claims appear
inconsistent:

(1) There is only one winning lottery number.

(2) The winning lottery number is 03.

(3) The winning lottery number is 61.

These three similar claims likewise seem inconsistent:

(1′) (the reductionist thesis) There is only one
physical-chemical realization of pain.

((2′) and (3′) reflect the multiple realizability
contention.) But there is no mystery in how to reconcile (1) –
(3). Append “per week” to (1), “this week” to (2), and “last week” to
(3). Similarly, append “per structure-type” to (1′), “in
humans” to (2′), and “in mollusks” to
(3′). Inconsistencies evaporate. Lewis's point is that
reductive identities are always specific to a domain.

Many reductionist philosophers have elaborated on Lewis's point with
scientific examples. Patricia Churchland (1986, Chapter 7), Clifford
Hooker (1981), Berent Enç (1983), and other philosophers of
science have described historical intertheoretic reductions where a
given reduced concept is multiply realized at the reducing level. A
common example is the concept of temperature from classical equilibrium
thermodynamics. Temperature in a gas is identical to mean molecular
kinetic energy. Temperature in a solid, however, is identical to mean
maximal molecular kinetic energy, since the molecules of a solid are
bound in lattice structures and hence restricted to a range of
vibratory motions. Temperature in a plasma is something else entirely,
since the molecular constituents of a plasma have been ripped apart.
Even a vacuum can have a (“blackbody”) temperature, though
it contains no molecular constituents. Temperature of classical
thermodynamics is multiply realized microphysically in a variety of
distinct physical states. Yet this is a “textbook” intertheoretic
reduction and cross-theory identification. The reductions and
identifications are specific to the domain of physical state.

Lewis's original insight also underlies Kim's (1989, 1992) appeals to
structure-specific “local reductions.” Kim agrees that multiple
realizability rules out a general reduction of (structure-independent)
psychology to physical science. But it permits (and even sanctions) a
local reduction to a theory of the physical mechanisms of a given
structure-type. (Kim admits that the relevant structure-types here
will probably be narrower than biological species.) Local reductions
involve “structure-specific bridge laws” where the mental-physical
biconditional occurs as the consequent of a conditional whose
antecedent denotes a specific structure-type (e.g., “if
X is a member of structure type S, then X
is in mental state M iff X is in physical state
P”). Conditionals whose antecedents denote different
structure types will typically have biconditionals as consequents
whose mental term- constituents are co-referential but whose physical
term- constituents denote different physical events. Multiple
realizability forces this much revision to the bridge laws of
classical reductionism. But according to Kim, local reductions are the
rule rather than the exception in science generally, and are
sufficient for any reasonable scientific or philosophical
purpose. Kim's approach is another way to express the tacit domain
specificity in scientific reductions.

Kim (1992) suggests and Bickle (1998, Chapter 4) emphasizes that
guiding methodological principles in contemporary neuroscience assume
continuity of underlying neural mechanisms. This assumption informs
most experimental techniques and theoretical conclusions drawn from
experimental results. Continuity is assumed both within and across
species. If radical multiple realizability really obtained among
species in the actual world, contemporary neuroscientific experimental
techniques built upon this assumption should bear little fruit. Why
study the macaque visual system to investigate human visual processing,
for example, if we can't safely assume some continuity across species?
Why should positron emission tomography (PET) and functional magnetic
resonance imaging (fMRI) reveal common areas of high metabolic activity
during psychological task performance, both across and within
individual humans—now down to a millimeter of spatial resolution?
Standard neuroscientific experimental procedures and even clinical
diagnostic tools would be hopelessly naïve in the face of
significant multiple realizability. But these procedures and tools do
work (and are not hopelessly naïve).

Kim and Bickle insist that these successes are evidence that
psychological functions are not as radically multiply realized as
functionalists and anti-reductionists suggest. Even neural plasticity
is systematic. It has a regular progression following damage to a
principal structure; there are underlying neural mechanisms that
subserve it. Furthermore, function following damage is often seriously
degraded. Persons can still talk, manipulate spatial representations,
or move their extremities, but their performance is often qualitatively
and quantitatively less than normal. This fact gives rise to tricky
questions about individuation of psychological function. Are these
alternative neural structures realizing the same psychological
function—the same mental kind—as before? (This
last response has been further developed. See the next section
below.)

Bechtel and Jennifer Mundale (1999) provide the most extensive
empirical details about hypothesized or assumed brain type-identities
across species in neuroscientific practice. Their explicit target is a
methodological consequence sometimes drawn from the multiple
realizability premise: if psychological states are multiply realized
across biological species, then neuroscience—the scientific
study of brains—will be of little use toward understanding
cognition. But as details of the neuroscience of vision demonstrate,
neuroscientists have successfully used understanding of the brain to
decompose cognitive visual function. The neuroscientific goal has
been to

show how functional considerations get built into
developing the structural taxonomy and how that taxonomy in turn can be
a heuristic guide in developing information-processing models. This
project has not been impaired by multiple realization of psychological
states; rather, it relies on the assumption that there is a common
realization of mechanisms for processing visual information across
species. (1999, 201)

It is difficult to argue with the empirical successes that have
obtained. So even if one accepts the multiple realizability contention,
one should be hesitant to draw strong consequences about
psychology's methodological autonomy from it.

In recent years critics have begun to challenge the truth of the
multiple realizability premise. One approach challenges the way that
mental kinds are individuated by multiple realizability proponents.

Nick Zangwill (1992) was the first to suggest that multiple
realizability across biological species has never “been
proven.” The multiple realizability contention assumes a
type-identity of mental kinds across species. According to Zangwill
this assumption is problematic, given that the obvious sensory and
motor differences across species by themselves yield different
cause-and-effect patterns at all but the grossest level of description.
If successful, this challenge undercuts the multiple realizability
argument by denying that the same mental kinds obtain across species,
to be realized by different physical mechanisms.

Lawrence Shapiro (2000) also contends that philosophers are too
quick to claim that a given kind is multiply realized. Some properties
of the realizers are relevant to the purposes, activities, or
capacities that define a given functional kind, but others are not.
Consider corkscrews. That functional kind can be “multiply
realized” in two tokens that differ only in their color. That
physical difference does not make them genuinely different realizations
of corkscrew, however, because it makes no difference to their
performance as corkscrews. Similarly for two corkscrews that differ
only in that one is made of aluminum and the other of steel. Although
that compositional difference might matter for some functional kinds,
it doesn't for corkscrews. As Shapiro notes, “steel and
aluminum are not different realizations of a waiter's
corkscrew because, relative to the properties that make them suitable
for removing corks, they are identical” (2000, p. 644).
Establishing genuine multiple realizability takes
argument—one must point to property differences in the
realizers that make for a functional difference.

Shapiro argues that this requirement sets up a dilemma. Consider
what appears to be a genuine case of multiple realizability, that is,
two objects that “do the same thing” but in very different
ways. Either the realizing kinds genuinely differ in their causally
relevant properties or they do not. If they do not, then we don't
really have a case of multiple realizability (like the
corkscrews that differ only in color or composition). If they do, then
they are different kinds. But then they are not the same kind and again
we don't have an instance of multiple realizability—of a
single kind with distinct realizations.

The usual justification for grouping distinct realizers under a
single functional kind is that the classification reveals interesting
similarities, of the sort we expect to be captured by laws or
generalizations of higher level science. But according to Shapiro, when
the realizing kinds differ significantly in their causally relevant
properties for the function at issue, any shared laws or
generalizations are “numbingly dull” (2000, p. 649): e.g.,
all realizers of mouse traps are used to catch mice; both camera eyes
and compound eyes have the function to see. Shapiro remarks: “If
[functional kinds] share many causally relevant properties, then they
are not distinct realizations … If they have no or only few
causally relevant properties in common, then there are no or just a few
laws that are true of both of them” (2000, p. 649). The first
horn acknowledges a single functional kind but denies that it is
multiply realized. The second undercuts the principal reason for
grouping genuinely different physical kinds under a single functional
kind. Shapiro concludes that taken together these two horns blunt any
claim of multiple realizations of the same functional
type.

Mark Couch (2004) presses a similar dilemma. Defending a claimed
multiple realization involves two steps. Proponents must show (i) that
the physical states (of the realizers) are type distinct, and (ii)
that the functional properties are type identical. Challenges to
claimed multiple realizations can attack either step and, most
importantly, the step challenged can differ from case to case.
(Successfully challenging either blocks the multiple realizability
argument.) As we saw in the previous section, Bechtel and Mundale
(1999) describe cases in which cognitive neuroscientists treat the
physical realizers (brain states) as type-identical across species
(attacking step i). In other cases—Couch's example is primate
versus octopus eyes—one can appeal to easily-found differences
in functional properties (attacking step ii). The two eyes have
different visual pigments in their photoreceptors, different retinas,
and different ways of focusing light. These physical differences lead
to straightforward input-output (functional) differences: in the optic
stimuli the two eyes respond to, in reaction times, and more. Their
functions may be similar, but similarity isn't identity and multiple
realization requires the latter. Cross-species functional
similarities are often quite superficial, especially across species
from widely differing taxa (a point shared by Couch and Shapiro). In
actual scientific practice, discovered physical (neural) differences
typically incline psychologists to seek out functional
differences. Couch's point is that the individuation of psychological
states, like the individuation of brain states, is an empirical
issue. Shapiro and Couch hint that claims to multiple realizability
rely heavily on “folk” psychological intuitions about
individuating mental kinds.

Bechtel and Mundale (1999) note that multiple realizability
proponents appeal to different amounts of “granularity” in
individuating mental and neurobiological kinds. Proponents are content
to analyze psychological states at a coarse-grained level, in which
only the loosest input-output similarities across species are
sufficient for mental kind identities. Yet they insist on very
fine-grained individuation for brain states, in which small differences
across species are sufficient for neural type-difference. But
psychological ascriptions admit of finer grains and neural ascriptions
admit of coarser grains. Bechtel and Mundale insist that when a common
grain is chosen for both, mental-neural type-identities holding across
species are found. In any case, it is unfair to hold neural
type-individuation to a very fine grain, while adopting a very coarse
grain for mental type-individuation.

Some of these arguments quickly attracted critical attention. For
example, Gillett (2003) argues that Fodor and other proponents of
multiple realization assume a ‘Dimensioned’ view of
realization that allows realizer/realized properties to be instantiated
in the distinct individuals that bear part-whole relations.
Shapiro and other recent challengers assume a ‘Flat’ view
of realization, which demands that realizer/realize properties be
instantiated in the same individual. Gillett shows first that
critical arguments like Shapiro's do not go through under the
Dimensioned view of realization; and second, that the critics have not
defended the Flat view over the Dimensioned view. Gillett concludes
that failing to directly address the nature of realization relation
vitiates critiques like Shapiro's and others, who are simply left
begging the question against original defenses of multiple realization
like Fodor's.

Besides his appeal to species-specific bridge laws and local
reducibility, Kim (1992) offers two additional replies to the multiple
realizability argument. His “denying projectibility” reply starts from
the familiar fact that the kind “jade” fragments into jadeite and
nephrite. Jade is thus incapable of passing the projectibility test for
nomicness because of its genuinely disjunctive nature. Multiple
realizability of psychological kinds yields the same consequence.
Instead of rendering psychology an autonomous special science, multiple
realizability implies that there is no structure-independent scientific
psychology. There are only “local” scientific psychologies, each
reducible to the theory of the underlying physical mechanisms of the
structure-type in question.

Closely related is Kim's “causal powers” reply. Scientific
kinds are individuated by their causal powers, and the causal powers of
each instance of some realized kind are identical to those of its
realizer. From these principles it follows that instances of a mental
kind with different physical realizations are distinct kinds. Thus
(structure-independent) mental kinds are not causal kinds, and hence
are disqualified as proper scientific kinds. Multiple realizability
yields the failure of structure-independent mental kinds to meet the
standards of scientific kinds. Notice that Shapiro's
“dilemma” (discussed in section 2.4 above) is in the spirit
of Kim's “causal powers” argument.

Kim's argument has attracted critical attention. As part of their
defense of the autonomy of the mental, Louise Antony and Joseph Levine
(1997) insist that it is not vast multiple realizability that makes a
property unprojectible—for a property like “having mass of
one gram” is the former but isn't the latter. The projectibility
of a nomic property only guarantees the projectibility of shared
properties that “are constitutive of or nomically
connected to it” (p. 90, authors' emphases). This renders
Kim's appeal to the jade analogy problematic for mental
properties. Block (1997) argues that kind-ness is both relative and
graded, and so projectibility is always with respect to particular
types of properties. Specifically, Block distinguishes two types: D
properties, which are selected (though not necessarily selected for)
and whose physical realizations are subject to constraints imposed by
laws of nature; and realization properties, which are due to the
peculiarities of some specific realization. Block argues that Kim's
unprojectibility argument is correct (and important) for realization
properties of mental kinds; but there are also genuine D properties of
mental kinds (not yet well understood) and these do project from one
realization to others, even in light of vast realization
differences.

Fodor (1997) distinguishes disjunctive from multiply
realized properties. The former, like jade, are neither
projectible nor nomic; but the latter, like mental properties (as
construed by functionalists) are both. Kim's analogy between jade
and pain breaks down, and thus so does his conclusion about the
unprojectibility of the latter. This undercuts the remaining steps in
his argument for reduction. Gene Witmer, on the other hand, (2003)
accepts Kim's “linking hypothesis” connecting the
unprojectibility of the disjunctive sum of physical realizers with the
unprojectibility of the functional kind itself. Instead he challenges
the unprojectibility of the disjunctive sum. There are categories whose
instances share nothing in common except abstract relational features
whose denoting expressions occur in generalizations that are
confirmable by their positive instances (the key feature of
projectibility). Witmer cites examples like “papers written after
brainstorming,” “products produced by the same
company,” and “a good night's sleep.” An
argument might overturn our intuitive verdict of confirmability by
positive instances for generalizations containing these terms but the
burden is on the denier. This is Witmer's “Moorean”
premise: “it is a Moorean fact that we have good reason to
believe, on the basis of a number of positive instances,
generalizations about pain” (67). Now the linking hypothesis
turns Kim's argument on its head. By modus tollens, the
disjunctive sum of physical realizers of pain is likewise projectible.
(Witmer also provides various readings of Kim's
“Inexplicability argument” based the causal exclusion
principle and argues that each reading fails.)

The more radical type of multiple realizability seems to force
increasingly narrower domains for reductions to be relativized—at
the extreme, to individuals at times. This much “local
reduction” seems inconsistent with the assumed generality of
science. To avoid this problem, some philosophers have suggested more
revolutionary changes to the “accepted” account of
(intertheoretic) reduction.

Following suggestions by Clifford Hooker (1981) and Enc (1983),
Bickle (1998, Chapter 4) argues that the radical type of multiple
realizability (in the same token system over times) is a feature of
accepted historical cases of scientific reduction. It even obtains in
the “textbook” reduction of classical equilibrium
thermodynamics to statistical mechanics and microphysics. For any token
aggregate of gas molecules, there is an indefinite number of
realizations of a given temperature—a given mean
molecular kinetic energy. Microphysically, the most fine-grained
theoretical specification of a gas is its microcanonical ensemble, in
which the momentum and location (and thus the kinetic energy) of each
molecule are specified. Indefinitely many distinct microcanonical
ensembles of a token volume of gas molecules can yield the same
mean molecular kinetic energy. Thus at the lowest level of
microphysical description, a given temperature is vastly multiply
realizable in the same token system over times. Nevertheless, the case
of temperature is a textbook case of reduction. So this type of
multiple realizability is not by itself a barrier to reducibility.

To accommodate this feature, Hooker (1981, Part III) supplements his
general theory of reduction with an account of
“token-to-token” reductions. His supplement builds the
possibility of multiple realizability (including the strong type)
directly into the definition of the reduction relation. Let S
be the predicate, “satisfies functional theory
F,” T be the class of systems to which the
token system in question belongs, S′ be an appropriate
predicate in some lower level theory of T-system causal mechanisms,
and T* be the class of systems to which S′
applies. Then, according to Hooker, “systems of type S
of class T are contingently token/token identical with
systems of type S′ in class T*
=df every instance (token) of a type S
system externally classified as in class T is contingently
identical with some instance (token) of a type S′
system externally classified as in class T*” (1981,
p. 504). By “externally classified,” Hooker refers to the
sort of cross-classification that holds across different
determinable/determinate hierarchies.

To address some acknowledged shortcomings in Hooker's
formulation of his general theory of reduction, Bickle (1998)
reformulates Hooker's insights (including his token-token
reduction supplement) within a set-theoretic “semantic”
account of theory structure and relations. The technical details are
complex and don't bear repeating here, but the basic idea is
straightforward. Bickle's “new wave” account
construes intertheoretic reduction as the construction of an image of
the set-theoretic structure of the models of the reduced theory within
the set comprising the models of the reducing, modulo a number of
conditions on the resulting mapping. Elements of the sets of models
include token real-world systems to which the theories apply (the
theories' “intended empirical applications”).

Other new conceptions of both reduction and the mind-brain identity
theory have been proposed. Elliott Sober (1999) insists that a
reductionist thesis actually follows from the multiple realizability
premise. He begins by attacking Putnam's (1967)
“objective” account of superior explanation, namely that
one explanation is superior to another if the former is more general.
According to Putnam, superior explanations “bring out the
relevant laws.” But Sober reminds us that explanatory
generalizations at lower levels bring out more details. Science
“aims for depth as well as breadth” and there is no
“objective rule” concerning which endeavor is
“better” (1999, 550). Both reductionists and
anti-reductionists err in privileging one aim at the expense of the
other. Sober then notes that multiple realizability
presupposes some form of asymmetric determination: the lower level
physical properties present at a given time determine the higher level
properties present. But this assumption commits its proponents to the
causal completeness of physics (a doctrine that Sober sketches toward
the end of his 1999). If one is also concerned with causal
explanation—if one holds that singular occurrences are explained
by citing their causes—then the causal completeness of physics in
turn commits multiple realizability proponents to physics'
possessing an important variety of explanatory completeness that all
other sciences lack. This is “reductionism of a sort”
(1999, 562).

William Bechtel and Robert McCauley (1999) develop a version of
“heuristic” mind-brain identity theory (HIT) and defend it
explicitly against multiple realizability. HIT insists that identity
claims in science typically are hypotheses adopted in the course of
empirical investigations, which serve to guide subsequent research.
They are not conclusions reached after empirical research has been
conducted. Concerning the multiple realizability of psychological on
brain (physical) states, cognitive neuroscience's heuristic
identity claims assert type-commonalities in comparative studies across
species, not type-differences. Bechtel and McCauley illustrate their
hypothesis with case studies: Brodmann's early 20th
century work mapping the brain into functionally relevant areas;
Ferrier's late-19th century work employing electrical
stimulation to cortex; and more recent detailed maps of visual
processing regions in the primate brain. All of these landmark
functional anatomical studies used multiple species. As Bechtel and
McCauley remind us,

when they consider theories of mind-brain relations,
philosophers seem to forget that the overwhelming majority of studies
have been on non-human brains. … Although the ultimate objective
is to understand the structure and function of the human brain,
neuroscientists depend upon indirect, comparative procedures to apply
the information from studies with non-human animals to the study of the
human brain. (1999, 70–71)

Heuristic psychoneural type-identity claims across species are key
components of these standard neuroscientific procedures.

Thomas Polger (2004) handles multiple realizability by developing a
“non-reductive mind-brain identity theory.” He insists that
appeals to stronger kinds of multiple realizability are only plausible
under prior commitment to functionalism, and so beg the question if
employed against the identity theory. Weaker claims can be handled in a
fashion akin to Bechtel's, McCauley's, and Mundale's
strategy: “the fact—if it is a fact—that many
different systems can have the same kinds of mental states does not
show that they do not all do so in virtue of having something in
common” (2004, 10). (One can plausibly read Bechtel, McCauley,
and Mundale as providing the empirical details for Polger's
assertion of realization-level commonality.) For the remaining
(moderate) forms of multiple realizability, Polger insists:
“either the [cognizing] thing shares some properties in common
[with us] or else it does not have [our] mental states after all”
(2004, 11). Polger adopts a reply akin to Couch's to
functionalist “empathetic” intuitions that we share mental
states with a wide range of terrestrial creatures: he denies that we
really attribute the same (as compared to similar) mental states to
other species (2004, 15).

In response to recovery of function following massive brain trauma,
Polger adopts Bechtel and Mundale's line (and perhaps
Bickle's): “Rather than supporting multiple realizability,
these cases suggest that we do not understand how the brain
works—how to individuate brain processes, events, states, and
properties” (2004, 17). In response to “standard”
multiple realization claims, he avails himself of Bechtel and
Mundale's “different grains” response (2004, 21–26).
In the end, Polger countenances some multiple realizability, but argues
that this much does not threaten his “nonreductive” version
of mind-brain identity theory:

Particular kinds of sensations, S1, …,
Sr, are identical to particular kinds of brain
states, B1, …,
Br. Sensation kinds may cluster into coarser, more
general species-specific mental state kinds, … but insofar as
they do, we expect that their members will share physical properties
… Creatures that are similar physically … may also have
relatively similar mental state kinds. … We should expect human
beings and higher primates to have similar conscious mental states
because their brains are quite similar to our own. And we should
expect the experiences of octopi or aliens to be different from ours
to the extent that their brains are quite different from our
own. (2004, 30)

When reduction or identity theory gets reconceived in ways built to
accommodate multiple realizability, are reductionists/identity
theorists and their critics simply talking past one another? It is
worth reminding ourselves that many nonreductive physicalists have
employed multiple realizability to argue against all forms of
psychophysical reductionism. If better general accounts of scientific
reduction or identity theory make room for multiple realizability,
these demonstrations count against this broader challenge. (If
“nonreductive” physicalism were to oppose only a specific
brand of psychophysical reductionism, that would weaken the position
significantly, so that it would be compatible with other forms of
“reductive” physicalism.) In fact, this broader challenge
to psychophysical reductionism traces back to Fodor (1974). While his
arguments explicitly targeted a reductionism built on the classical
Nagelian account, Fodor suggested in footnote 2 that “what I
shall be attacking is what many people have in mind when they refer to
the unity of science, and I suspect (though I shan't try to prove
it) that many of the liberalized versions of reductionism suffer from
the same basic defect as what I shall take to be the classical form of
the doctrine.”

In searching for reductive unity underlying the variety of cognitive
systems, Paul Churchland (1982) once recommended descending
“below” neurobiology and even biochemistry, to the level of
nonequilibrium thermodynamics. He insisted that finding reductive unity
there was more than a bare logical possibility because of some
parallels between biological processes, whose multiply realized kinds
find reductive unity there, and cognitive activity (especially
learning).

Concerning Pylyshyn's (1984) attack on reductionist
methodology, Patricia Churchland (1986, Chapter 9) suggests that
functional theories are constructed in lower level sciences. New levels
of theory thus get inserted between those describing the structure of
the lower level kinds and those of purely functional kinds: between,
for example, the physiology of individual neurons and cognitive
psychology. We might find a common neurofunctional property for a given
type of psychological state across a wide variety of distinct brains.
And if the scope of the macro-theory doesn't extend beyond that
of its microfunctional counterpart, then reduction will be achieved
despite vast multiple realizability at the microstructural level.
Neurocomputational approaches that have blossomed since the early 1990s
give real empirical credence to Churchland's suggestion.

Bickle (2003) claims that if we leave our neuroscientific
understanding at the systems level, psychoneural multiple realizability
seems obvious. Neural systems differ significantly across species. But
neuroscience does not stop at the systems level. As it moved further
down, into cellular physiology and increasingly the molecular biology
of nervous tissue, type-identities across species have been found. Many
molecular mechanisms of neural conductance, transmission, and
plasticity are the same in invertebrates through mammals. This matters
for psychology because mechanisms of cognition and consciousness are
increasingly being found at these levels. Bickle's key example is
memory consolidation, the conversion of labile, easily disrupted
short-term memories into more durable, stable long-term form. Work with
fruit flies, sea slugs, and mice has revealed the role of the cyclic
adenosine monophosphate (cAMP)-protein kinase A (PKA)-cAMP
responsive-element binding protein (CREB) signaling pathway in key
forms of experience-driven synaptic plasticity. Across these very
distinct taxa, this molecular circuitry has also been implicated
experimentally in memory consolidation. By altering a single protein in
this cascade (using biotechnology and molecular genetics),
experimenters have built mutant organisms whose short-term memory
remains intact (as does their sensory, motor, and motivational
capacities), but which cannot consolidate these short-term memories
into long-term form. Bickle quotes with approval statements like the
following, from insect biologists Josh Dubnau and Tom Tully:

In all systems studied, the cAMP signaling cascade has been
identified as one of the major biochemical pathways involved in
modulating both neuronal and behavioral plasticity. … More
recently, elucidation of the role of CREB-mediated
transcription in long-term memory in flies, LTP and long-term memory in
vertebrates, and long-term facilitation in A. californica [a
sea slug] suggest that CREB may constitute a universally conserved
molecular switch for long term memory (1998, 438).

Memory consolidation is just one psychological phenomenon, and so
its ruthless reduction to molecular events doesn't establish a
general claim about unitary mechanisms across widely divergent taxa for
other shared cognitive kinds. For that argument, Bickle turns to
principles of molecular evolution. The first principle holds that
evolution at the molecular level—changes to the amino acid
sequence of a given protein—is much slower in functionally
important (“constrained”) domains than in functionally less
important ones. The second principle is that molecular evolution is
much slower in all domains of “housekeeping” proteins,
especially in ones that participate in cell-metabolic processes in many
tissue types. These two principles imply that these molecules, their
domains, and the intracellular processes they participate in will
remain constant across existing biological species that share the
common ancestor first possessing them. (This is what Dubnau and Tully
refer to above as a “universally conserved” molecular
switch.) In the end, any psychological kind that affects an
organism's behavior must engage the cell-metabolic machinery in
individual neurons. In the brain, causally speaking, that's where
the rubber meets the road. But that's the machinery conserved
across existing biological species—changes to it, especially its
functionally constrained domains, have (almost) inevitably been
detrimental to an organism's survival. So we should expect that
the molecular mechanisms for any causally efficacious cognitive kind be
“universally conserved.” The discovery of these shared
mechanisms of memory consolidation is not some isolated case, but
follows from the core principles of molecular evolution. As
‘molecular and cellular cognition’ proceeds, we should
expect more examples of unitary realizers (reductions) of shared
psychological kinds

At present, nonreductive physicalism (probably) is still the
dominant position in Anglo-American philosophy of mind. Its proponents
continue to appeal to the standard multiple realizability argument (see
section 1 above) to challenge all versions of psychophysical
reductionism and identity theory. However, the recent challenges over
the past decade have attracted some notice. Versions of type-identity
theory and reductive physicalism have made comebacks (Gozzano and Hill,
2012). Perhaps the nonidentity of mental content properties with any
physical properties is no longer “practically received
wisdom,” as Ernest LePore and Barry Loewer called it more than
two decades ago?

Criticisms of these new challenges are also starting to amass. Carl
Gillett and Ken Aizawa have been the most vocal recent defenders of
multiple realizability. Gillett (2003) develops a precise framework for
understanding compositionality relations in science generally, and uses
this framework to define property realization and multiple realization,
and to distinguish two senses of realization. The first sense,
“flat” realization, involves both realized and realizing
properties inhering in a single object. The second sense,
“dimensioned” realization, involves realized and realizing
properties inhering in distinct individuals standing in a compositional
relationship. This distinction is important for two reasons, according
to Gillett (2002, 2003). First, scientific explanations employ
dimensioned realizations, as inter-level mechanistic explanations
relate distinct individuals, Second, Fodor and other proponents of the
standard multiple realizability argument assumed a dimensioned account.
But the arguments of Shapiro and other recent critics (see section 2
above) challenge the existence of multiple realizability only by
assuming flat realization, and no recent critic has defended flat
realization as the correct account involved in the scientific cases at
issue.

Applying Gillett's precise framework explicitly, Ken Aizawa
and Gillett defend the existence of multiple realization in a variety
of sciences (2009a) and “massive multiple realization”
about human psychological properties at every level of organization,
from the structure and function of proteins in neurons to social
interactions (2009b). Their detailed focus in the latter essay is
visual processing. They contend that neuroscientists, unlike
philosophers, are unfazed by massive multiple realization. Multiple
realization has been so contentious in philosophy of mind, they insist,
because philosophers both tacitly assume flawed accounts of realization
like the flat view, and due to an accepted narrative linking multiple
realization to the strict methodological autonomy of psychology from
neuroscience. Aizwa and Gillett (2009b) conclude, however, that the
empirical details of vision research shows that a co-evolutionary
research methodology is not just consistent with, but explicitly
motivated by massive multiple realization. So this narrative not only
helps to blind philosophers to facts that scientists recognize as
unproblematic; it is also empirically false.

More recently, Aizawa and Gillett (2011) distinguish two strategies
scientists might adopt to deal with putative cases of multiple
realization. One strategy is simply to take multiple realization at
face value. The other is to split the higher-level multiply realized
kind into a variety of sub-kinds, one for each of its distinct lower
level realizers, and then eliminate the original higher-level kind, at
least for the purposes of further scientific investigation and
development. Do scientists always favor the second strategy, as recent
philosophical critics of multiple realizability would seem to
recommend? With its well-known distinctions between different types or
systems, memory research would seem regularly to have employed this
“eliminate-and-split” strategy. Yet Aizawa and Gillett
argue that such an assessment oversimplifies the actual scientific
details in even these much-discussed cases. Here too they sense an
important general methodological lesson: psychology takes account of
neuroscience discoveries, so even taking multiple realization at face
value does not imply strict methodological autonomy. But the actual
details of how psychology takes neuroscientific discoveries into
account depends both on the nature of the psychological kinds in
question and the needs of theorizing specific to psychology.

Aizawa has also challenged many of the specific recent challenges to
the standard multiple realizabilty argument. After separating three
distinct arguments in Bechtel and Mundale (1999) (discussed in section
2 above), Aizawa (2008) sets his critical sights on their Central
Argument, which argues against multiple realization from the existence
and continued success of brain mapping studies. He argues that Bechtel
and Mundale misrepresent the actual nature of these studies and
methods employed in functional localization studies. Working with
exactly the scientific examples Bechtel and Mundale discuss (mostly
from the functional neuroanatomy of vision) Aizawa argues that claims
about psychological functions do not play the specific role in these
studies that Bechtel and Mundale insist, and so the success of these
studies does not imply the falsity of multiple realization. Later in
that paper Aizawa challenges two of Bechtel and Mundale's key
insistences. He denies that if psychological properties were multiply
realized, then functional taxonomy of the brain would have to be
carried out independently of psychological function. And he denies
that multiple realization rules out comparisons of brains across
different species. Hence all the premises of Bechtel and Mundale's
Central Argument are false. Aizawa (2007) criticizes Bickle's (2003)
argument (discussed in section 2 above) that a unitary realization of
memory consolidation across species has been found at the level of
molecular mechanisms, despite widespread neural differences in these
brains at higher levels of neuroscientific investigation. According to
Aizawa, the protein components of these evolutionarily conserved
molecular mechanisms, and the molecular-genetic components coding for
them, are themselves multiply realized. Finally, Aizawa (forthcoming)
presents numerous scientific examples of multiple realization by
“compensatory differences.” In such cases, changes to one
or more properties that jointly realize a realized property G are
compensated for by changes in others of the jointly realizing
properties. Although his overall goal in this paper is to bring this
form of multiple realization to wider recognition and study by
philosophers of science, he uses the broader “Gillett-Aizawa
framework” to argue that highly specific determinate properties,
not just generic determinable properties, are multiply realized in
this specific fashion. Such multiply realized determinate properties
are indeed exactly similar across distinct realizations, and so answer
the dilemma posed by Shapiro and others (discussed extensively in
section 2 above).

Recent critics of the standard multiple realizability argument have
likewise not been quiet. Lawrence Shapiro (2008) raises some
methodological difficulties involved in testing whether a given
psychological kind actually is multiply realized. (For a related
argument see Thomas Polger 2009.) Shapiro reminds us of the crucial
role that auxiliary assumptions play in hypothesis testing generally
(within a broadly hypothetico-deductive model), and considers a
collection of explicit auxiliary assumptions that might be implicitly
used to establish a multiple realization hypothesis. He presents a
recent ferret brain-rewiring experiment as a scientific example (in
which axonal inputs from the primary visual tract were redirected in
ferret embryos to project to primary auditory cortex—see Sharma
et al. 2000 for the actual scientific details). An auxiliary
hypothesis requiring multiply realized higher-level (in this case,
psychological) kinds to be “exactly
similar”—identical—across distinct realizers
won't help the proponent of the standard multiple realizability
argument with this purported case.. It is easy to measure better visual
performance in the normally-wired control ferrets compared to the
re-wired experimental animals. While the experimentally re-wired
animals have some visual function, it is diminished significantly
compared to controls. “Exact similarity” (identity) of
visual function is thus not present across these groups. On the other
hand, one might argue for the multiple realizability premise in this
ferret re-wiring case using an auxiliary hypothesis that only requires
similarity in multiply realized higher level properties, yet still
requires that differences across the realizers should not be limited
only to the differences that cause differences in the realized (in this
case, visual) properties. (Shapiro remarks that this auxiliary
assumption seems best to capture the sense of multiple realization
stressed by proponents of the standard argument.) But if we adopt it,
again the ferret re-wiring case seems not to provide an empirical
instance of multiple realization. Shapiro remarks: “the
differences in ferret brains explain nothing more than differences in
ferret visual properties” (2008, 523). Shapiro also argues that
his detailed discussion of hypothesis testing difficulties for any
multiple realization hypothesis reveals a flaw in Bechtel and
Mundale's (1999) influential criticism (discussed in section 2
above). Bechtel and Mundale's examples, drawn from the
comparative functional neuroanatomy of vision, only compare homologous
brain structures. But these only have differences that make a
difference in their visual properties, nothing else. Instead of these
examples, Shapiro insists, “one should be looking at different
brains that reveal similar visual properties despite their
differences” (2008, 524)—exactly the kinds of evidence that
Bechtel and Mundale's emphasis on homologies doesn't
consider.

Shapiro and Polger (2012) build upon their accounts of the complexity
of actually testing for scientifically-justified multiple
realization. They insist that it renders the significance of multiple
realization far more dubious than philosophers of mind typically
suppose. They introduce explicit criteria to capture the common
assumption that multiple realization requires not merely differences
between realizing kinds, but “differently the same”-ness:
the features of entities A and B that lead them to be
classified differently by the realizing science S2
“must be among those that lead them to be commonly
classified” by the realized science S1 (2012,
282, criterion iii). This explicit criterion rules out a common appeal
to camera eyes with different photoreceptive chemicals in their
retinal cones from being genuine (empirical) instances of multiple
realization. Considered coarsely, such eyes are doing the same thing
in the same way, so they're not “differently the same.”
Considered finely, the two kinds of eyes are sensitive to different
ranges and peaks of spectral stimulation, so they're
“differently different, not differently the same” (2012,
283–284).

Shapiro and Polger's final explicit criterion captures the
“differently the same” intuition in terms of quanitative
differences: the relevant variation between entities A and B in
realizing science S2 “must be greater than” the
individual differences between A and B recognized by realized science
S1 (2012, 282, criterion iv). The variation
recognized by the realizing science must not merely map onto
individual differences between A and B recognized by the realized
science. The demands in actually establishing multiple realization are
thus quite strict. Not any old variation will do. According to Shapiro
and Polger, these strict demands show both that multiple realization
in the sense required to fund the standard argument is “a
relatively rare phenomenon”—despite the vast variability
everywhere in the world—and that a “relatively
modest” mind-brain identity theory has little actually to worry
about from it (2012, 284).

Similar in some ways to Couch's arguments (discussed in
section 2 above), Colin Klein recently raises a challenge to scientific
contributions made by multiply realized kinds. Noting the varieties of
things that materials science classifies as ‘brittle,’
Klein (2008) notes that few to none of the many scientific discoveries
about realization-restricted brittle things—about brittle steel,
for example—generalize to other realization-restricted types
(like brittle glass). Klein insists that generalizations about genuine
scientific kinds should be projectable across instances of those kinds,
so this requirement seems not to be met by a significant class of
multiple realized kinds (the realization-restricted ones). Applying
this point to psychological kinds, instead of supporting a
scientifically-backed nonreductive physicalism, it appears rather that
special sciences should abandon multiply realized kinds. Klein notes
that proponents of scientifically-based multiple realizability can find
terms in special sciences that figure in legitimate explanations, and
so appear to refer to projectable multiply realize kinds. But close
investigation of some paradigmatic examples reveals these to be
idealizations of actual kinds. Special-science kind-terms are thus
typically ambiguous. Sometimes a given term refers to an actual but
realization-restricted kind. Other times it refers to features of
explanatory but non-actual idealized models. (Klein 2008 illustrates
this ambiguity with his detailed example from materials science.)
Neither suffices to provide a kind of actual multiple realization that
the standard argument requires. However, he insists that his argument
isn't entirely negative for non-reductive physicalists.
Idealizations can function in explanations that are autonomous in an
important sense from lower level sciences. And Kim's (1996)
assumption, that all explanatory work in science must appeal to
realization-restricted kinds and properties (discussed in section 2
above), is simply incorrect. Still, Klein insists, there appear to be
no actual and projectable—hence genuinely
scientific—multiple realized kinds.

Finally, Bickle (2010) questions whether the “second
wave” of criticisms of the standard argument, those that
challenge the multiple realization premise itself (discussed in section
2 above), give aid and comfort to psychoneural reductionists. Since
psychoneural reductionism was one of the explicit targets of the
standard multiple realization argument, one might plausibly assume that
they do. Yet none of the “second wavers” are themselves
reductionists (with the possible exception of Shapiro, and more
recently Bechtel 2009, although his response to the multiple
realization argument figures nowhere in his plumb for
“mechanistic reduction”). Some (Polger 2004) are explicitly
anti-reduction. (Though Polger 2004 is also explicitly
anti-anti-reduction. He argues that multiple realization has little if
anything to do with reduction.) This fact alone should give a
psychoneural reductionist pause. Second, the direction that the second
wave debates have developed, starting with Gillett's (2003)
criticisms—deeply into the nature of the realization relation,
and so into the metaphysics of science rather than into science
itself—should prompt the psychoneural reductionist with a
metascientific bent to simply tell the second wavers, thanks for
nothing! Does that leave psychoneural reductionism back on its heels,
in light of the standard multiple realization argument? Not at all,
Bickle (2010) insists. For the “first wave”
actual-scientific-history challenge to the first premise of the
standard argument, and the initial critical discussions in section 2
above) turns out never to have been rejoined by anti-reductionists. Why
not? Bickle speculates that Kim's more metaphysically-inspired
challenge to the standard argument was the culprit. Non-reductive
physicalists seem to have assumed that rejoining Kim's argument
dismisses the entire first wave of challenges. It does not. There are
numerous examples of multiply realized kinds that are components of
scientific theories widely acknowledged to having been reduced to other
theories. So multiple realizaation alone is no barrier to actual
scientific reduction. The detailed scientific cases that fill in that
“first wave” challenge to the standard argument remain
unanswered to this day.

So the renewed critical interest in multiple realizability, begun
more than a decade ago, continues to the present day. The assumption
that multiple realizability “seals the deal” against
reductive physicalism and the type identity theory of mind was
misplaced initially, and is now even more misplaced after the second
wave of recent criticisms. Proponents of the standard argument need to
follow Aizawa's and Gillett's recent leads, and offer new
defenses and counter-responses. What is at stake here should not be
underemphasized: nothing less than one of the most influential
arguments from late-20th century Anglo-American philosophy,
one that impacts not only the philosophical mind-body problem but also
the relationship between sciences addressing higher and lower levels of
the universe's organization.

Aizawa, Kenneth and Carl Gillett, 2011. “The Autonomy of
Psychology in the Age of Neuroscience,” in P.M. Illari, F. Russo,
and J. Williamson (eds.), Causality in the Sciences, New York:
Oxford University Press, 203–223.

Bechtel, William and Robert McCauley, 1999. “Heuristic
Identity Theory (or Back to the Future): the Mind-Body Problem Against
the Background of Research Strategies in Cognitive
Neuroscience,” Proceedings of the 21st Annual
Meeting of the Cognitive Science Society, Mahwah, NJ: Lawrence
Erlbaum Associates.

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